This invention relates to a control system for a sewing machine and the like, wherein thread being fed to the machine is monitored, and when the rate of thread feed decreases below a predetermined value, the operation of the machine is terminated.
In some continuous sewing operations, where the attendant does not physically operate the sewing machine but is required only to monitor the sewing machine operation along with the operation of other equipment, a sewing machine may accidentally continue to operate without properly sewing the work piece. For example, when the bottom looper thread gives out on a chain stitch sewing machine, or when the bobbin thread of a bobbin-type sewing machine gives out, it is difficult for an attendant to visually detect the depletion of the thread since the thread is on the bottom surface of the work product. If the bottom looper thread breaks or is depleted the thread being fed to the needle will form an incomplete top stitch in the work product. This means that the needle thread will continue to be fed to the sewing machine in spite of the fact that the bottom looper thread has broken or has been depleted and the attendant observing the machine operation is not likely to immediately detect the the depletion of the looper thread since the needle of the machine continues to insert the thread into the upper surface of the work product. In this situation the thread being fed through the sewing machine needle is not engaged by the bottom looper thread, and the needle thread tends to pull partially out of the hole formed by the sewing machine needle when the needle withdraws from the work product, and in most instances, the upper portion of the stitch made in the work product will remain in the work product. This results in a smaller amount of needle thread being used in the work product since portions of the thread are pulled out by the retraction of the needle from the work product, which results in the needle thread being fed at a slower rate to the sewing machine.
Other examples of a decrease in needle thread rate which indicate a sewing machine malfunction would be a decrease in stitch length as when the work piece is hung in the delivery mechanism or there is excessive drag on the work piece, a decrease in stitch depth as when the machine begins to sew through fewer plys of material, a break in the monitored thread as when the needle thread breaks, or when the machine skips stitches.
While various attempts have been made to control sewing machines and to terminate the operation of sewing machines in response to thread breakage or depletion of thread supply, the various known prior art devices monitor the tension of the thread being fed to a sewing machine or monitor the speed of operation of the sewing machine. These prior art devices apply more tension to the thread and occasionally vary the tension of the thread being fed to the sewing machine, resulting in a higher likelihood of thread breakage, or require more power to operate the sewing machine. Also, the known prior art devices generally cannot detect a malfunction in machine operation by monitoring the feed rate of a thread to a machine.